Abstract (English)

Temperature is one of the most important environmental factors for ectotherms and the temperature experienced during development affects many important life history traits in insects. The eusocial honeybees maintain a more or less constant temperature for their brood (34-35°C) via active thermoregulation. Small deviations from this optimal temperature range affected the synaptic organization in ...

Abstract (English)

Temperature is one of the most important environmental factors for ectotherms and the temperature experienced during development affects many important life history traits in insects. The eusocial honeybees maintain a more or less constant temperature for their brood (34-35°C) via active thermoregulation. Small deviations from this optimal temperature range affected the synaptic organization in the brain, behavioural performance, and learning and memory in adult honeybees. Larger deviations led to malformations and increased mortality. In contrast to honeybees, most bee species are solitary. Their offspring are largely exposed to the fluctuating ambient temperatures. If solitary bees showed the same negative effects of deviations from the optimal temperatures during development as honeybees, they would suffer huge fitness losses. Thus, solitary bees should have adapted to fluctuating and changing temperatures during development.

In this thesis, we investigated potential influences of temperature during development on body size, development time, synaptic organization in the brain, and cognitive abilities of the solitary Red mason bee, Osmia bicornis (Hymenopter, Megachilidae). We used three fluctuating (10-25, 15-30, and 20-35°C) as well as three constant temperature treatments (17.5, 22.5, and 27.5°C) plus one control group as a reference for natural temperature conditions. Moreover, we varied the duration of exposure to the different temperature treatments: In one year, O. bicornis offspring were exposed to the experimental temperatures during their entire development. In the following year, the respective temperature treatments were only applied during development inside the cocoon.

In general, body size decreased with increasing temperature during larval development. Bees attained higher prepupal weights under fluctuating conditions than in the correspondent constant temperature treatment. Increasing and fluctuating (vs. constant) temperatures accelerated development in almost all stages and temperature regimes. However, the prepupal stage was prolonged in the warm temperature treatments, but only in bees that had experienced these temperatures during entire development. Thus, the extension of the prepupal phase might be a mechanism to adjust adult eclosion to the onset of wintering temperatures when larval development was accelerated due to hot summers.

Regarding potential temperature effects on the brain of O. bicornis, we focused on the synaptic organization in the calyces of the mushroom bodies (MBs). The MBs are prominent neuropils in the insect brain that play an important role for learning, memory, and orientation. We analysed temperature effects on neuropil size as well as density and number of distinct synaptic complexes, the microglomeruli (MG), in the calyces and compared our results with data on honeybees. Temperature affected neuropil size, but this effect was largely compensated by a reciprocal effect on MG density – but only in bees that had experienced the experimental temperatures during entire development. As a result, overall MG numbers were hardly affected by developmental temperature in these bees. However, though we did not detect such a compensatory effect on MG density in bees that were exposed to the experimental temperatures only during post-larval development, the temperature effects on the brain, particularly on overall MG number, were considerably and significantly smaller in all experimental groups of O. bicornis than in honeybees.

To test whether temperature during post-larval development might influence the learning abilities of the bees, we developed a visual learning paradigm for O. bicornis. We found evidence for an innate preference for blue, but the bees were generally able to overcome this innate preference via conditioning. We did not detect an effect of developmental temperature on the abilities for visual learning and fast reversal learning.

Our studies on O. bicornis revealed considerable plasticity in body size and development time in response to different temperature treatments during development. In contrast, brain development and learning abilities seem to be largely buffered against temperature influences. Our results suggest that O. bicornis might use temperature during larval development as a cue for temperature conditions in their environment that allows the adjustment of several physiological processes in subsequent developmental stages to the prevailing conditions. Our results support the hypothesis that, in contrast to honeybees, solitary bees are adapted and, thus, less susceptible to variable and fluctuating temperatures during development.